Radioactivity has long been used to determine the density of materials. The simplest method is the measurement of the attenuation of a beam of photons passing through the material of interest. For example, in the measurement of the density of soil, a source of gamma rays and a detector are lowered into the ground in two different boreholes. In such a case, the intensity registered by the detector is I=I.sub.o e.sup.-uL where I.sub.o is the intensity before absorption, L is the distance between the source and the detector, and u the absorption coefficient of the soil. Thus u is a measure of the number of electrons present in a unit volume, and therefore an indication of the density.
Very often, however, the material of interest cannot be placed between the source and the detector. In such a case gamma scattering methods must be used. A collimated source and a detector are placed near the surface of the material and photons are directed from the source toward the material. Some of these photons are backscattered toward the detector and those backscattered photons (Compton scattering) are detected and registered by the detector. Since the number of backscattered photons depends on the electron density of the material, a count thereof is a measure of the density of the material.
Such gamma scattering methods are often necessary to obtain information regarding geological strata penetrated by boreholes in oil exploration and recovery operations. Knowing the densities of the various formations in the borehole is of value in identifying the particular formations. If the rock matrix is known, a knowledge of the formation density enables the determination of the porosity of the formation. Thus, the use of these density logs with other seismograph and gravity meter data will reveal some of the major changes in the lithology.
Gamma well logging methods employing density logging tools have been commonly used to measure the density of formations penetrated by the borehole. Density logging tools utilize a gamma ray source and a gamma ray detector shielded from each other to prevent the detector from counting the radiation that is emitted directly from the radioactive source towards the detector. During operation of the tool, gamma rays (or photons) emitted from the radiation source enter the borehole to be studied, and interact with the atomic electrons of the material around the borehole by photoelectric absorption, Compton scattering, or (if the energy of the gamma radiation is high enough) by pair production. The photoelectric absorption process, as well as the pair production process, remove from the original gamma ray beam those particular photons that are involved in these reactions. In the Compton scattering process the involved photon only loses some of its energy while changing its original direction of travel. The amount of energy lost by the photon is a function of the scattering angle.
Some of the photons emitted from the radioactive source into the sample are scattered towards the detector. Many of these never reach the detector, since their direction is changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process or the pair production process. The scattered photons that reach the detector and interact with it are counted by the electronic equipment associated with the detector.
Generally, density logging tools take advantage of the Compton scattering effect, where gamma radiation is scattered in an interaction with free or loosely bound electrons in the scattering medium. The gamma radiation scattering thus achieved can be measured in that when the energy of the incident gamma radiation is known, the energy of the gamma radiation that is Compton scattered at a particular angle with respect to the incident radiation, can be calculated. Furthermore, the probability that a gamma ray will be scattered from a particular zone or volume of an earth formation is proportional to the number of electrons in the zone. The count rate of detected, singly scattered gamma radiation is, therefore, related to the density of the scattering zone.
Both the scattering in the material around the borehole as well as the absorption of photons from the original and the scattered gamma ray beams are functions of the electron density of the material which, in turn, can be interpreted in terms of the density of the material itself. For short source detector spacings, the scattering process dominates over the absorption process, and the counting rate of the detector increases with increasing density. For long source detector spacings, the absorption process dominates over the scattering process, and the counting rate of the detector decreases with increasing density of the sample.
The usefulness of density logging tools for obtaining indications of earth formation density surrounding a borehole is well known, and the principles on which they operate are, similarly, well known to those skilled in the art. However, the physics of such tools suitable for making a count of backscattered photons are very complicated because the detector registers photons of different energies (due to different scattering angles) and the photons are attenuated during their passage through the material. The attenuation of the photons is also a function of the electron density of the material. The situation becomes even more complicated when the material of interest is not directly accessible, as when other substances, such as a casing or well fluid, are placed between the material and the measuring instrument. In such cases, the measured data cannot be interpreted universally and individual calibration curves must be taken for each condition.
In the logging of formations traversed by a well bore, it has long been recognized that irregularities and the diameter and shape of the well bore affect the physical measurements being made therein. Thus, it is frequently necessary to make a caliper survey of the diameter of the well bore to provide a correct interpretation of the physical measurement along any particular portion of the borehole.
Commonly, radioactive well logging in cased, air or gas filled boreholes requires either special equipment or at a minimum, extensive corrections to the log produced by a tool intended for use under fluid-filled borehole standard conditions. Generally, the magnitude of the correction required increases with the inner diameter of the borehole. The change from a liquid filled into a substantially gas filled borehole, however, is frequently such a significant departure from the design conditions of the tool that correction of the log with exceptional accuracy is usually not possible, regardless of the borehole inner diameter.
Other major difficulties encountered in using conventional density logging tools and methods are the disturbing effects of undesired, interfering materials and noise.
Density logging tools for investigating the bulk density of boreholes in earth formations are not sufficiently sensitive when used to locate the interface of two materials having little difference in their densities. Such is the case, for example, in mining and storage wells in salt formations.
In the mining of a typical well in a salt formation, a borehole is drilled in the salt formation and one or more casing strings are cemented in the borehole. Inner and outer tubing strings are run into the innermost casing with hydrocarbons pumped down the annulus between the outer tubing and inner tubing. Such hydrocarbons form a pad over the roof of the cavern caused by the mining operation. Fresh water is pumped down the inner tubing to form brine and the brine is forced up the annulus formed between the outer and inner tubings. It often becomes necessary to locate the interface between the hydrocarbons and brine and possibly the interface between the brine and water if the operation has been stopped to permit the brine-water interface to form. Such interfaces are difficult to detect using density logging tools.
Large quantities of product are now being stored in salt caverns formed by mining operations. Often a number of storage wells are located in the same salt formation. Such storage wells will be filled with product and brine which create a brine-product interface. A product, such as ethylene dichloride or caustic, has a density very close to brine and therefore it is very difficult to detect the interface. Storage wells for ethylene lichloride or caustic will also have a hydrocarbon pad. Thus when it becomes necessary to determine the level of product in such a storage well, prior art tools have difficulty locating the brine-product interface.
Further, in such mining and storage wells it often is necessary to determine other media and borehole anomalies such as the location of various sized casing and tubing strings, casing collars, etc.
Prior logging tools are not sufficiently sensitive for such applications. The present invention overcomes the deficiencies of such prior logging tools by providing a substantially improved instrument and method. The improved instrument and method provide a more sensitive reading, thereby greatly enhancing detection. Other objects and advantages of the invention will appear from the following description.